Display device comprising semiconductor layer having LDD regions

ABSTRACT

To provide a semiconductor device including a narrowed bezel obtained by designing a gate driver circuit. A gate driver of a display device includes a shift register unit, a demultiplexer circuit, and n signal lines. By connecting the n signal lines for transmitting clock signals to one stage of the shift register unit, (n−3) output signals can be output. The larger n becomes, the smaller the rate of signal lines for transmitting clock signals which do not contribute to output becomes; accordingly, the area of the shift register unit part is small compared to a conventional structure in which one stage of a shift register unit outputs one output signal. Therefore, the gate driver circuit can have a narrow bezel.

This application is a continuation of copending U.S. application Ser.No. 16/850,401, filed on Apr. 16, 2020 which is a continuation of U.S.application Ser. No. 15/366,368, filed on Dec. 1, 2016 (now U.S. Pat.No. 10,629,149 issued Apr. 21, 2020) which is a continuation of U.S.application Ser. No. 14/959,777, filed on Dec. 4, 2015 (now U.S. Pat.No. 9,514,696 issued Dec. 6, 2016) which is a continuation of U.S.application Ser. No. 14/325,603, filed on Jul. 8, 2014 (now U.S. Pat.No. 9,208,742 issued Dec. 8, 2015, which are all incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine, ormanufacture. The present invention particularly relates to, for example,a semiconductor device, a display device, a light-emitting device, apower storage device, a driving method thereof, or a manufacturingmethod thereof. In particular, the present invention relates to, forexample, a semiconductor device including a transistor and a method formanufacturing the semiconductor device.

2. Description of the Related Art

As a means of downsizing, weight saving, and obtaining a narrowed bezelof a flat panel display typified by a liquid crystal display device anda light-emitting display device, it is known that a gate driver and apixel portion are formed on one substrate. To obtain a bezel which isfurther narrowed, downsizing of the gate driver is required. Maincircuits of the gate driver include a shift register.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2002-49333

SUMMARY OF THE INVENTION

A reduction in width of a shift register which is a main circuit of agate driver is an effective means of obtaining a narrowed bezel becausethe reduction in width of the shift register leads to a reduction inwidth of the whole of a gate driver circuit.

One object of one embodiment of the present invention is to provide agate driver circuit in which the width of a shift register unit part isreduced. Another object of one embodiment of the present invention is toprovide a semiconductor device in which the width of a gate drivercircuit is reduced without increasing the delay time of a signal of asignal line of the gate driver circuit. Another object of one embodimentof the present invention is to provide a semiconductor device in which agate driver circuit is designed to achieve a narrowed bezel.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a driver circuit whichincludes a shift register unit, a demultiplexer circuit electricallyconnected to the shift register unit, and n signal lines (n is a naturalnumber of four or more). The shift register unit is electricallyconnected to one or more of the n signal lines. The demultiplexercircuit is electrically connected to one to (n−3) of the n signal lines.

Another embodiment of the present invention is a driver circuit whichincludes m shift register units (m is a natural number of three ormore), m demultiplexer circuits electrically connected to the m shiftregister units, and n signal lines (n is a natural number of four ormore). Each of the m shift register units is electrically connected toone or more of the n signal lines. Each of the m demultiplexer circuitsis electrically connected to one to (n−3) of the n signal lines. To oneof the m shift register units, one of outputs of a demultiplexer circuitelectrically connected to a shift register unit in the previous stage ofone of the m shift register units is input. To one of the m shiftregister units, one of outputs of the demultiplexer circuit electricallyconnected to the shift register unit in the next stage of the one of them shift register units is input.

Another embodiment of the present invention is a driver circuitincluding a shift register unit including a set signal line, a firsttransistor, a second transistor, a third transistor, a fourthtransistor, a fifth transistor, and a sixth transistor; a demultiplexercircuit; and n signal lines (n is a natural number of four or more). Inthe driver circuit, one of a source and a drain of the first transistoris electrically connected to a high power supply potential line; theother of the source and the drain of the first transistor iselectrically connected to one of a source and a drain of the secondtransistor and the demultiplexer circuit; a gate of the first transistoris electrically connected to the set signal line; the other of thesource and the drain of the second transistor is electrically connectedto a low power supply potential line; a gate of the second transistor iselectrically connected to the demultiplexer circuit, one of a source anda drain of the fourth transistor, one of a source and a drain of thefifth transistor, and one of a source and a drain of the sixthtransistor; one of a source and a drain of the third transistor iselectrically connected to the high power supply potential line; theother of the source and the drain of the third transistor iselectrically connected to the other of the source and the drain of thefourth transistor; a gate of the third transistor is electricallyconnected to one of the n signal lines; a gate of the fourth transistoris electrically connected to one of the n signal lines; the other of thesource and the drain of the fifth transistor is electrically connectedto the low power supply potential line; a gate of the fifth transistoris electrically connected to the set signal line; the other of thesource and the drain of the sixth transistor is electrically connectedto the high power supply potential line; a gate of the sixth transistoris electrically connected to a reset signal line; the demultiplexercircuit includes a buffers (a is a natural number of one or more and(n−3) or less); each of the a buffers is electrically connected to theother of the source and the drain of the first transistor and the gateof the second transistor; the a buffers are electrically connected tothe respective n signal lines; and each of the a buffers includes anoutput terminal.

According to one embodiment of the present invention, a semiconductordevice with a narrowed bezel can be formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a block diagram and a circuit diagram illustratingone embodiment of a semiconductor device.

FIG. 2 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 3A to 3E are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device.

FIGS. 4A to 4D are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device.

FIG. 5 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 6 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 7A to 7C illustrate electronic devices each including asemiconductor device which is one embodiment of the present invention.

FIGS. 8A to 8C illustrate an electronic device including a semiconductordevice which is one embodiment of the present invention.

FIG. 9 illustrates an overall view of a gate driver circuit.

FIGS. 10A and 10B illustrate a shift register unit.

FIGS. 11A and 11B illustrate another shift register unit which is adummy stage.

FIGS. 12A and 12B illustrate a demultiplexer.

FIGS. 13A and 13B illustrate a demultiplexer.

FIG. 14 illustrates a buffer.

FIGS. 15A and 15B illustrate another shift register unit.

FIGS. 16A and 16B illustrate another shift register unit which is adummy stage.

FIGS. 17A and 17B illustrate another buffer.

FIGS. 18A and 18B illustrate a way to obtain a narrower bezel.

FIG. 19 is a timing diagram of a shift register unit.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that the mode and details can be variouslychanged without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments. Inaddition, in the following embodiments, the same portions or portionshaving similar functions are denoted by the same reference numerals orthe same hatching patterns in different drawings, and descriptionthereof will not be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such a scale.

In addition, terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can bereplaced with each other in this specification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or a voltage, and a potential and a voltage are usedas synonymous words in many cases. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

In this specification, in the case where an etching step is performedafter a photolithography step, removal of a mask formed in thephotolithography step that is performed after the etching step is notdescribed in some cases for simplicity.

Embodiment 1

In this embodiment, a structure of a semiconductor device that is oneembodiment of the present invention and a method for manufacturing thesemiconductor device are described with reference to drawings.

FIG. 1A illustrates a liquid crystal display device as one example of asemiconductor device. The liquid crystal display device in FIG. 1Aincludes a pixel portion 101, a gate driver 104, a source driver 106, mscan lines 107 which are arranged in parallel or substantially inparallel and whose potentials are controlled by the gate driver 104, andn signal lines 109 which are arranged in parallel or substantially inparallel and whose potentials are controlled by the source driver 106.Further, the pixel portion 101 includes a plurality of pixels 301arranged in a matrix. Capacitor lines 115 which are arranged in parallelor substantially in parallel to the scan lines 107 are also provided.The capacitor lines 115 may be arranged in parallel or substantially inparallel to the signal lines 109. The gate driver 104 and the sourcedriver 106 are collectively referred to as a driver circuit portion insome cases.

Each scan line 107 is electrically connected to the n pixels 301 in thecorresponding row among the pixels 301 arranged in m rows and n columnsin the pixel portion 101. Each signal line 109 is electrically connectedto the m pixels 301 in the corresponding column among the pixels 301arranged in m rows and n columns. Note that m and n are each an integerof 1 or more. Each capacitor line 115 is electrically connected to the npixels 301 in the corresponding row among the pixels 301 arranged in mrows and n columns. Note that in the case where the capacitor lines 115are arranged in parallel or substantially in parallel to the signallines 109, each capacitor line 115 is electrically connected to the mpixels 301 in the corresponding column among the pixels 301 arranged inm rows and n columns.

FIG. 1B illustrates a circuit structure that can be used for the pixel301 in the liquid crystal display device illustrated in FIG. 1A.

The pixel 301 illustrated in FIG. 1B includes a liquid crystal element132, a transistor 131, and a capacitor 133.

The potential of one of a pair of electrodes of the liquid crystalelement 132 is set according to the specifications of the pixels 301 asappropriate. The alignment state of the liquid crystal element 132depends on written data. A common potential may be applied to one of thepair of electrodes of the liquid crystal element 132 included in each ofthe plurality of pixels 301. Further, the potential supplied to one ofthe pair of electrodes of the liquid crystal element 132 in the pixel301 in one row may be different from the potential supplied to one ofthe pair of electrodes of the liquid crystal element 132 in the pixel301 in another row. Alternatively, in the IPS mode or the FFS mode, oneof the pair of electrodes of the liquid crystal element 132 can beconnected to a capacitor line CL.

As examples of a driving method of the liquid crystal display deviceincluding the liquid crystal element 132, any of the following modes canbe given: a TN mode, an STN mode, a VA mode, an ASM (axially symmetricaligned micro-cell) mode, an OCB (optically compensated birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(antiferroelectric liquid crystal) mode, an MVA mode, a PVA (patternedvertical alignment) mode, an IPS mode, an FFS mode, a TBA (transversebend alignment) mode, and the like. Other examples of the driving methodof the liquid crystal display device include an ECB (electricallycontrolled birefringence) mode, a PDLC (polymer dispersed liquidcrystal) mode, a PNLC (polymer network liquid crystal) mode, and aguest-host mode. Note that one embodiment of the present invention isnot limited to this, and various liquid crystal elements and drivingmethods can be used as a liquid crystal element and a driving methodthereof.

The liquid crystal element may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less and is optically isotropic; therefore,alignment treatment is not necessary and viewing angle dependence issmall.

In the pixel 301 in the mth row and the nth column, one of a sourceelectrode and a drain electrode of the transistor 131 is electricallyconnected to a signal line DL_n, and the other is electrically connectedto one of a pair of electrodes of the capacitor 133 and the other of thepair of electrodes of the liquid crystal element 132. A gate electrodeof the transistor 131 is electrically connected to a scan line GL_m. Thetransistor 131 has a function of controlling whether to write a datasignal by being turned on or off.

The other of the pair of electrodes of the capacitor 133 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as capacitor line CL). The potential of the capacitor lineCL is set according to the specifications of the pixel 301 asappropriate. The capacitor 133 functions as a storage capacitor forretaining written data. Note that in the IPS mode or the FFS mode, theother of the pair of electrodes of the capacitor 133 can be electricallyconnected to one of the pair of electrodes of the liquid crystal element132.

For example, in the liquid crystal display device including the pixel301 of FIG. 1B, the pixels 301 are sequentially selected row by row bythe gate driver 104, whereby the transistors 131 are turned on and adata signal is written.

When the transistors 131 are turned off, the pixels 301 in which thedata has been written are brought into a holding state. This operationis sequentially performed row by row; thus, an image is displayed.

Note that in this specification and the like, examples of liquid crystaldisplay devices having liquid crystal elements are a transmissive liquidcrystal display device, a transflective liquid crystal display device, areflective liquid crystal display device, a direct-view liquid crystaldisplay device, and a projection liquid crystal display. An example ofthe liquid crystal element is an element that controls transmission ornon-transmission of light by optical modulation action of liquidcrystals. The element can include a pair of electrodes and a liquidcrystal layer. The optical modulation action of liquid crystal iscontrolled by an electric field applied to the liquid crystal (includinga horizontal electric field, a vertical electric field and a diagonalelectric field). Note that specifically, the following can be used for aliquid crystal element, for example: a nematic liquid crystal, acholesteric liquid crystal, a smectic liquid crystal, a discotic liquidcrystal, a thermotropic liquid crystal, a lyotropic liquid crystal, alow-molecular liquid crystal, a high-molecular liquid crystal, a polymerdispersed liquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, a banana-shaped liquidcrystal, and the like.

Instead of a liquid crystal display device, one example of asemiconductor device can be a display element, a display device, alight-emitting device, and the like. A display element, a display devicewhich is a device including a display element, a light-emitting element,and a light-emitting device which is a device including a light-emittingelement can employ various modes and can include various elements.Examples of a display element, a display device, a light-emittingelement, or a light-emitting device include an LED (e.g., a white LED, ared LED, a green LED, or a blue LED), a transistor (a transistor whichemits light depending on current), an electron emitter, a liquid crystalelement, electronic ink, an electrophoretic element, a grating lightvalve (GLV), a plasma display panel (PDP), a micro electro mechanicalsystem (MEMS), a digital micromirror device (DMD), a digital microshutter (DMS), MIRASOL (registered trademark), an interferometricmodulator display (IMOD), a piezoelectric ceramic display, or a carbonnanotube, which are display media whose contrast, luminance,reflectivity, transmittance, or the like is changed by electromagneticaction. Examples of display devices having electron emitters include afield emission display (FED) and an SED-type flat panel display (SED:surface-conduction electron-emitter display). Display devices havingelectronic ink or electrophoretic elements include electronic paper andthe like.

Next, a specific example of a liquid crystal display device including aliquid crystal element in the pixel 301 is described. FIG. 2 is across-sectional view for illustrating a cross-sectional structure of theliquid crystal display device. FIG. 2 illustrates cross-sectionalstructures of a gate driver and a pixel circuit. In this embodiment, asa semiconductor device, a liquid crystal display device of a verticalelectric field mode is described.

In the liquid crystal display device described in this embodiment, aliquid crystal element 209 is provided between a pair of substrates (asubstrate 200 and a substrate 242).

The liquid crystal element 209 includes a conductive layer 206 over thesubstrate 200, films controlling alignment (hereinafter referred to asalignment films 251 and 252), a liquid crystal layer 207, and aconductive layer 208. The conductive layer 206 functions as oneelectrode of the liquid crystal element 209, and the conductive layer208 functions as the other electrode of the liquid crystal element 209.

Thus, “liquid crystal display device” refers to a device including aliquid crystal element. The liquid crystal display device includes adriver circuit for driving a plurality of pixels, for example. Theliquid crystal display device may also be referred to as a liquidcrystal module including a control circuit, a power supply circuit, asignal generation circuit, a backlight module, and the like providedover another substrate.

The liquid crystal display device shown in FIG. 2 is provided with atransistor 211 included in a pixel portion 220 and a transistor 221included in a driver circuit portion 230 over the substrate 200.Furthermore, the liquid crystal element 209 including the conductivelayer 206, the liquid crystal layer 207, and the conductive layer 208 isincluded in the pixel portion 220.

In the liquid crystal display device shown in FIG. 2 , the transistor211 provided in the pixel portion 220 includes a semiconductor layer 212in which a channel region is formed, and the transistor 221 provided inthe driver circuit portion 230 includes a semiconductor layer 222 inwhich a channel region is formed.

Here, components of the display device shown in FIG. 2 are describedbelow.

An insulating film 201 and an insulating film 202 are formed over thesubstrate 200. The semiconductor layer 212 and the semiconductor layer222 in which channel regions of the transistors are formed are formed inisland-like shapes over the insulating film 202.

There is no particular limitation on the property of a material and thelike of the substrate 200 as long as the material has heat resistanceenough to withstand at least heat treatment to be performed later. Forexample, a glass substrate, a ceramic substrate, a quartz substrate, asapphire substrate, or the like may be used as the substrate 200.Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like may be used as thesubstrate 200. Furthermore, any of these substrates further providedwith a semiconductor element may be used as the substrate 200. In thecase where a glass substrate is used as the substrate 200, a glasssubstrate having any of the following sizes can be used: the 6thgeneration (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm),and the 10th generation (2950 mm×3400 mm). Thus, a large-sized liquidcrystal display device can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 200,and the transistor may be provided directly on the flexible substrate.Further alternatively, a separation layer may be provided between thesubstrate 200 and the transistor. The separation layer can be used whenpart or the whole of an element portion formed over the separation layeris completed and separated from the substrate 200 and transferred toanother substrate. In such a case, the transistor can be transferred toa substrate having low heat resistance or a flexible substrate as well.

Each of the insulating film 201 and the insulating film 202 can have asingle layer structure or a stacked-layer structure including aninsulating film formed of silicon oxide, silicon oxynitride, siliconnitride, or the like by a chemical vapor deposition (CVD) method, asputtering method, a thermal oxidation method, or the like. As acombination of the insulating film 201 and the insulating film 202, acombination of silicon oxynitride and silicon oxide can be used forexample.

The semiconductor layer 212 and the semiconductor layer 222 arepreferably formed using crystalline silicon, but may be formed usingamorphous silicon. Crystalline silicon is formed in such a manner that,after an amorphous silicon film is formed, the amorphous silicon film iscrystallized by laser irradiation. Alternatively, after a metal filmsuch as a Ni film is formed over an amorphous silicon film, theamorphous silicon film may be thermally crystallized. Furtheralternatively, a crystalline silicon film may be formed by a CVD method.

An insulating film 231 is a gate insulating film. The insulating film231 can be formed with a single layer structure or a stacked-layerstructure using an insulating film of silicon oxide, silicon oxynitride,silicon nitride, or the like by a CVD method, a sputtering method, orthe like.

Further, when a silicon oxide film is formed by a CVD method using anorganosilane gas as the insulating film 231, the crystallinity of asemiconductor film to be formed later can be increased, whereby theon-state current and the field-effect mobility of the transistor can beincreased. As an organosilane gas, a silicon-containing compound such astetraethoxysilane (TEOS) (chemical formula: Si(OC₂H₅)₄),tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC₂H₅)₃), ortrisdimethylaminosilane (SiH(N(CH₃)₂)₃) can be used.

Alternatively, the insulating film 231 may be formed by performingplasma treatment on the semiconductor layers 212 and 222 to oxidize ornitride the surfaces of the semiconductor layers 212 and 222. Forexample, the insulating film 231 is formed by plasma treatment with amixed gas of a rare gas such as He, Ar, Kr or Xe, and oxygen, nitrogenoxide (NO₂), ammonia, nitrogen, hydrogen, or the like. In this case,when excitation of plasma is performed by introducing a microwave,plasma with a low electron temperature and high density can begenerated. The surface of a semiconductor film can be oxidized ornitrided by oxygen radicals (OH radicals are included in some cases) ornitrogen radicals (NH radicals are included in some cases) generated bythis high-density plasma.

By treatment using such high-density plasma, an insulating film with athickness of 1 nm to 20 nm inclusive, typically 5 nm to 10 nm inclusive,is formed over the semiconductor film. Since reaction in this case issolid-phase reaction, interface state density between the insulatingfilm and the semiconductor film can be made quite low. By such highdensity plasma treatment, since the semiconductor film is directlyoxidized (or nitrided), variation in thickness of the formed insulatingfilm can be made extremely small. By solid-phase oxidation of thesurface of the semiconductor film through such high-density plasmatreatment, an insulating film which has favorable uniformity and lowinterface state density can be formed.

As for the insulating film 231, only the insulating film formed by highdensity plasma treatment may be used, or one or more of insulating filmsof silicon oxide, silicon oxynitride, or silicon nitride may bedeposited and stacked by a CVD method, a sputtering method, or the like.In any case, when transistors include an insulating film formed byhigh-density plasma in a part or the whole of a gate insulating film,variations in characteristics can be reduced.

Next, a first conductive layer 272 and a second conductive layer 273 areformed over the insulating film 231. The first conductive layer 272 andthe second conductive layer 273 can be formed using an element selectedfrom among tantalum (Ta), tungsten (W), titanium (Ti), molybdenum (Mo),aluminum (Al), copper (Cu), chromium (Cr), niobium (Nb), and the like,or an alloy material or a compound material containing such an elementas a main component (for example, tantalum nitride). Alternatively, theyare each formed using a semiconductor material typified bypolycrystalline silicon doped with an impurity element such asphosphorus. Note that the first conductive layer 272 and the secondconductive layer 273 may be formed of the same conductive material ordifferent conductive materials.

As a combination of the first conductive layer 272 and the secondconductive layer 273, a combination of tantalum nitride and tungsten, acombination of tungsten nitride and tungsten, a combination ofmolybdenum nitride and molybdenum, or the like can be used for example.Here, the first conductive layer is formed to a thickness greater thanor equal to 20 nm and less than or equal to 100 nm by a CVD method, asputtering method, or the like. The second conductive layer is formed toa thickness greater than or equal to 100 nm and less than or equal to400 nm. Although a stacked-layer structure of two conductive films isused in this embodiment, one layer or a stacked-layer structure of threeor more layers may be alternatively used. In the case of a three-layerstructure, a stacked-layer structure of a molybdenum layer, an aluminumlayer, and a molybdenum layer may be employed.

Impurity regions 216 and impurity regions 217 are formed in thesemiconductor layer 212, and impurity regions 226 and impurity regions227 are formed in the semiconductor layer 222. An impurity element canbe introduced by an ion doping method, an ion implantation method, orthe like with the use of an n-type impurity element or a p-type impurityelement. As the n-type impurity element, phosphorus (P), arsenic (As),or the like can be used. As the p-type impurity element, boron (B),aluminum (Al), gallium (Ga), or the like can be used.

An insulating film 236 is an interlayer insulating film. Conductivelayers 218 and conductive layers 228 are source electrodes and drainelectrodes.

The insulating film 236 can be formed using an insulating film made ofsilicon oxide, silicon oxynitride, silicon nitride, or the like by a CVDmethod, a sputtering method, or the like. Alternatively, an organicresin such as an acrylic resin, a polyimide resin, abenzocyclobutene-based resin, a siloxane-based resin, a polyamide resin,or an epoxy resin can be used.

Each of the conductive layers 218 and the conductive layers 228 can havea single layer structure or a stacked-layer structure including oneelement selected from aluminum, tungsten, titanium, tantalum,molybdenum, nickel, and neodymium, or an alloy containing a plurality ofany of these elements. For example, a conductive layer that is formedusing an alloy that contains a plurality of any of the elements givenabove can be formed from an aluminum alloy that contains titanium, analuminum alloy that contains neodymium, or the like can be used. In thecase of forming a stacked-layer structure, for example, a structurewhere an aluminum layer or the above-described aluminum alloy layer isprovided between titanium layers can be used. Note that the conductivelayers 218 and 228 serve as source electrodes and drain electrodes ofthe transistors.

An insulating film 238 is an interlayer insulating film. The conductivelayer 206 formed over the insulating film 238 is a pixel electrode. Theconductive layer 206 serves as an anode or a cathode in a light-emittingdevice.

The insulating film 238 can be formed using an insulating film made ofsilicon oxide, silicon oxynitride, silicon nitride, or the like by a CVDmethod, a sputtering method, or the like. Alternatively, an organicresin such as an acrylic resin, a polyimide resin, abenzocyclobutene-based resin, a siloxane-based resin, a polyamide resin,or an epoxy resin can be used.

As the conductive layer 206, a transparent conductive film composed of alight-transmitting conductive material may be used, and indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, and the like can be used. Needless to say, it ispossible to use indium tin oxide, indium zinc oxide, indium tin oxidewith silicon oxide added, or the like. As the conductive layer 206, amaterial that has a high work function, for example, an element selectedfrom nickel (Ni), tungsten (W), chromium (Cr), platinum (Pt), zinc (Zn),tin (Sn), indium (In), or molybdenum (Mo), or an alloy materialincluding any of the metal elements as its main component, for example,titanium nitride, titanium silicon nitride, tungsten silicide, tungstennitride, tungsten silicide nitride, or niobium nitride may be used toform a single layer film or a film of stacked layers.

Reference numeral 251 denotes an alignment film. The alignment film 251can be formed using an organic resin such as polyimide. The thickness ofthe alignment film 251 is greater than or equal to 40 nm and less thanor equal to 100 nm, preferably greater than or equal to 50 nm and lessthan or equal to 90 nm. With such a thickness, the pretilt angle of aliquid crystal material can be made large, which can reducedisclination.

A film having a coloring property (hereinafter referred to as a coloringfilm 246) is formed on the substrate 242. The coloring film 246functions as a color filter. Further, a light-blocking film 244 adjacentto the coloring film 246 is formed on the substrate 242. Thelight-blocking film 244 functions as a black matrix. The coloring film246 is not necessarily provided in the case where the liquid crystaldisplay device is a monochrome display device, for example.

The coloring film 246 is a coloring film that transmits light in aspecific wavelength range. For example, a red (R) color filter fortransmitting light in a red wavelength range, a green (G) color filterfor transmitting light in a green wavelength range, a blue (B) colorfilter for transmitting light in a blue wavelength range, or the likecan be used.

The light-blocking film 244 preferably has a function of blocking lightin a particular wavelength range, and can be a metal film, an organicinsulating film including a black pigment, or the like.

An insulating film 248 is formed on the coloring film 246. Theinsulating film 248 functions as a planarization film or suppressesdiffusion of impurities in the coloring film 246 to the liquid crystalelement side.

The conductive layer 208 is formed on the insulating film 248. Theconductive layer 208 functions as the other of the pair of electrodes ofthe liquid crystal element in the pixel portion. Note that the alignmentfilm 251 is formed over the conductive layer 206, and the alignment film252 is formed on the conductive layer 208.

The liquid crystal layer 207 is formed between the conductive layer 206and the conductive layer 208. The liquid crystal layer 207 is sealedbetween the substrate 200 and the substrate 242 with the use of asealant (not illustrated). The sealant is preferably in contact with aninorganic material to prevent entry of moisture and the like from theoutside.

A spacer may be provided between the conductive layer 206 and theconductive layer 208 to maintain the thickness of the liquid crystallayer 207 (also referred to as a cell gap).

Next, a method for manufacturing the transistors 211 and 221 of theliquid crystal display device illustrated in FIG. 1A is described withreference to FIGS. 3A to 3E and FIGS. 4A to 4D.

First, the substrate 200 is prepared. Here, a glass substrate is used asthe substrate 200.

Next, the insulating film 201 and the insulating film 202 aresequentially stacked over the substrate 200. Each of the insulating film201 and the insulating film 202 can have a single layer structure or astacked-layer structure using an insulating film formed of siliconoxide, silicon oxynitride, silicon nitride, or the like by a CVD method,a sputtering method, a thermal oxidation method, or the like. As acombination of the insulating film 201 and the insulating film 202, acombination of silicon oxynitride and silicon oxide can be used forexample.

Next, the semiconductor film is formed over the insulating film 202 andis selectively etched to form the semiconductor layer 212 and thesemiconductor layer 222. The semiconductor layer 212 and thesemiconductor layer 222 are preferably formed using crystalline silicon.In this embodiment, after amorphous silicon is formed by a CVD method,the amorphous silicon is crystallized by laser irradiation. Note thatheat treatment for removing hydrogen may be performed before the laserirradiation (FIG. 3A).

Next, the insulating film 231 is formed to cover the semiconductor layer212 and the semiconductor layer 222. The insulating film 231 can beformed with a single layer structure or a stacked-layer structure usingan insulating film of silicon oxide, silicon oxynitride, siliconnitride, or the like by a CVD method, a sputtering method, a thermaloxidation method, or the like. Here, silicon oxide is used for the gateinsulating film (FIG. 3B).

Next, a first conductive film 292 and a second conductive film 293 aresequentially stacked over the insulating film 231. Each of the firstconductive film 292 and the second conductive film 293 can be formedusing an element selected from among tantalum (Ta), tungsten (W),titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), chromium(Cr), niobium (Nb), and the like, or an alloy material or a compoundmaterial containing such an element as a main component (for example,tantalum nitride). Alternatively, they are each formed using asemiconductor material typified by polycrystalline silicon doped with animpurity element such as phosphorus. Note that the first conductive film292 and the second conductive film 293 may be formed of the sameconductive material or different conductive materials. Here, the firstconductive film and the second conductive film are formed using atantalum nitride film and a tungsten film, respectively (FIG. 3C).

Then, resist masks 234 are selectively formed over the second conductivefilm 293, and first etching treatment and second etching treatment areperformed using the resist masks 234. By performing the first etchingtreatment, the first conductive film 292 and the second conductive film293 that are formed over the insulating film 231 are selectively etched.Thus, a stacked-layer structure including a first conductive layer 232 aand a second conductive layer 233 a that can serve as a gate electrodeis made to remain over the semiconductor layer 212, and a stacked-layerstructure including a first conductive layer 232 b and a secondconductive layer 233 b that can serve as a gate electrode is made toremain over the semiconductor layer 222 (FIG. 3D).

By performing the second etching treatment after that, end portions ofthe second conductive layer 233 a and the second conductive layer 233 bare selectively etched, thereby obtaining a structure where the secondconductive layer 233 a and the second conductive layer 233 b each have asmaller width than the first conductive layer 232 a and the firstconductive layer 232 b (FIG. 3E).

An etching method for carrying out the first etching treatment and thesecond etching treatment may be selected as appropriate. In order toimprove the etching rate, a dry etching apparatus using a high densityplasma source of ECR (electron cyclotron resonance) or ICP (inductivelycoupled plasma) or the like may be used. With appropriate control of theetching conditions of the first etching treatment and the second etchingtreatment, the end portions of the first conductive layers 232 a and 232b and the second conductive layers 233 a and 233 b can be formed intodesired tapered shapes.

Next, an impurity element is introduced into the semiconductor layer 212and the semiconductor layer 222 using the first conductive layer 232 a,the first conductive layer 232 b, the second conductive layer 233 a, andthe second conductive layer 233 b as masks to form low-concentrationimpurity regions 215 in the semiconductor layer 212 andlow-concentration impurity regions 225 in the semiconductor layer 222(FIG. 4A).

An impurity element can be introduced by an ion doping method, an ionimplantation method, or the like with the use of an n-type impurityelement or a p-type impurity element. As the n-type impurity element,phosphorus (P), arsenic (As), or the like can be used. As the p-typeimpurity element, boron (B), aluminum (Al), gallium (Ga), or the likecan be used.

Here, an example is described in which the impurity regions 215 areformed in regions which do not overlap with the first conductive layer232 a in the semiconductor layer 212; however, the impurity regions 215can be formed in regions overlapping with the first conductive layer 232a depending on the condition of introducing the impurity element.Further, an example is described in which the impurity regions 225 areformed in regions which do not overlap with the first conductive layer232 b in the semiconductor layer 222; however, the impurity regions 225can be formed in regions overlapping with the first conductive layer 232b depending on the condition of introducing the impurity element.

A resist mask 235 is selectively formed over the first conductive layer232 a, the second conductive layer 233 a, and the semiconductor layer212, and an impurity element is introduced into the semiconductor layers212 and 222 with the use of the resist mask 235, the first conductivelayer 232 b, and the second conductive layer 233 b as masks. As aresult, the impurity regions 216 and the impurity regions 217 are formedin the semiconductor layer 212, and the impurity regions 226 and theimpurity regions 227 are formed in the semiconductor layer 222. Notethat the impurity element goes through the first conductive layer 232 bto be introduced into the semiconductor layer 222 (FIG. 4B).

The impurity element can be introduced by an ion doping method, an ionimplantation method, or the like. As the n-type impurity element,phosphorus (P), arsenic (As), or the like can be used. As the p-typeimpurity element, boron (B), aluminum (Al), gallium (Ga), or the likecan be used. Here, the impurity regions 216, 217, 226, and 227 aresubjected to ion doping using phosphorus (P).

In the semiconductor layer 212, the high-concentration impurity regions217 formed in regions which are not covered with the resist mask 235serve as a source region and a drain region of the transistor, and thelow-concentration impurity regions 216 formed in regions which arecovered with the resist mask 235 and do not overlap with the firstconductive layer 232 a serve as LDD regions of the transistor. Further,in the semiconductor layer 222, the high-concentration impurity regions227 formed in regions which do not overlap with the first conductivelayer 232 b serve as a source region and a drain region of thetransistor, and the low-concentration impurity regions 226 formed inregions which overlap with the first conductive layer 232 b but do notoverlap with the second conductive layer 233 b serve as LDD regions ofthe transistor.

The LDD region is a region to which an impurity element is added at lowconcentration between a channel forming region and a source region or adrain region which is formed by being doped with an impurity element athigh concentration. By providing the LDD region, the effects of reducingthe electric field in the vicinity of the drain region and preventingdeterioration due to hot carrier injection can be obtained. In order toprevent deterioration of on-state current due to hot carriers, astructure (also referred to as a GOLD (gate-drain overlapped LDD)structure) may be employed in which a LDD region overlaps with a gateelectrode with a gate insulating film interposed therebetween. Thisembodiment shows an example in which a LDD region is used in then-channel transistor 211 included in the pixel portion and a GOLDstructure is used in the n-channel transistor 221 included in the drivercircuit portion. However, one embodiment of the present invention is notlimited thereto, and a GOLD structure may be used in the transistorincluded in the pixel portion 220.

Next, an interlayer insulating film is formed. Here, the insulating film236 is formed as the interlayer insulating film. Then, opening portionsare selectively formed in the insulating film 231 and the insulatingfilm 236, and the conductive layers 218 and the conductive layers 228that serve as source electrodes and drain electrodes are formed (FIG.4C).

The insulating film 236 can be formed using an insulating film made ofsilicon oxide, silicon oxynitride, silicon nitride, or the like by a CVDmethod, a sputtering method, or the like. Alternatively, an organicresin such as an acrylic resin, a polyimide resin, abenzocyclobutene-based resin, a siloxane-based resin, a polyamide resin,or an epoxy resin can be used. Here, the insulating film 236 is formedusing silicon oxide, silicon oxynitride, or silicon nitride by a CVDmethod.

The conductive layers 218 and the conductive layers 228 can be formedwith a single layer structure or a stacked-layer structure using oneelement selected from aluminum, tungsten, titanium, tantalum,molybdenum, nickel, and neodymium, or an alloy containing a plurality ofany of these elements. For example, a conductive layer that is formedusing an alloy that contains a plurality of any of the elements givenabove can be formed from an aluminum alloy that contains titanium, analuminum alloy that contains neodymium, or the like can be used. In thecase of forming a stacked-layer structure, for example, a structurewhere an aluminum layer or the above-described aluminum alloy layer isprovided between titanium layers can be used. Note that the conductivelayers 218 and 228 serve as the source electrodes and the drainelectrodes of the transistors.

Next, the insulating film 238 is formed. Then, an opening portion isprovided in the insulating film 238, and the conductive layer 206serving as a pixel electrode is formed so as to be electricallyconnected to the conductive layer 218. The conductive layer 206 servesas an anode or a cathode in a light-emitting device (FIG. 4D).

The insulating film 238 can be formed using an insulating film made ofsilicon oxide, silicon oxynitride, silicon nitride, or the like by a CVDmethod, a sputtering method, or the like. Alternatively, an organicresin such as an acrylic resin, a polyimide resin, abenzocyclobutene-based resin, a siloxane-based resin, a polyamide resin,or an epoxy resin can be used.

A transparent conductive film formed using a light-transmittingconductive material may be used for the conductive layer 206 whichserves as a pixel electrode, and indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, or the likecan be used. Needless to say, it is possible to use indium tin oxide,indium zinc oxide, indium tin oxide with silicon oxide added, or thelike. Furthermore, a material that has a high work function, forexample, an element selected from nickel (Ni), tungsten (W), chromium(Cr), platinum (Pt), zinc (Zn), tin (Sn), indium (In), or molybdenum(Mo), or an alloy material including any of the metal elements as itsmain component, for example, titanium nitride, titanium silicon nitride,tungsten silicide, tungsten nitride, tungsten silicide nitride, orniobium nitride may be used to form a single layer film or a film ofstacked layers.

Thus, the transistor 211 and the transistor 221 of the liquid crystaldisplay device illustrated in FIG. 1A can be manufactured.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Modification Example

A modification example of the gate electrode in Embodiment 1 isdescribed with reference to FIG. 5 .

In FIG. 2 , the gate electrode has a two-layer structure including theconductive films. In FIG. 5 , the gate electrode has a single-layerstructure including a conductive film 261.

Furthermore, impurity regions 266 and impurity regions 276 in FIG. 5 arelow-concentration impurity regions and serve as LDD regions oftransistors. Impurity regions 267 and impurity regions 277 in FIG. 5 arehigh-concentration impurity regions and serve as source regions anddrain regions of the transistors. The low-concentration impurity regions266 and 276 and the high-concentration impurity regions 267 and 277 areformed using a resist mask in a manner similar to that of thelow-concentration impurity regions 216 and the high-concentrationimpurity regions 217, which is illustrated in FIGS. 4A and 4B.

The transistors each including the gate electrode having a single-layerstructure can be manufactured by a simple process, whereby costreduction can be achieved.

Embodiment 2

In this embodiment, a structure of a semiconductor device different fromthat of the semiconductor device of Embodiment 1 is described withreference to the drawings.

In FIG. 6 , a transistor 811 is a transistor included in a pixelportion, and a transistor 821 is a transistor included in a drivercircuit portion.

As illustrated in FIG. 6 , a conductive layer 832 serving as a gateelectrode is formed over a substrate 800. An insulating film 831 servingas a gate insulating film is formed so as to cover the conductive layer832. A semiconductor layer 812 and a semiconductor layer 822 are formedover the insulating film 831. A channel region 816, a channel region826, impurity regions 817, and impurity regions 827 are formed in thesemiconductor layer 812 and the semiconductor layer 822. The impurityregions 817 and the impurity regions 827 serve as source regions anddrain regions.

As the substrate 800, the substrate 200 described in Embodiment 1 can beused as appropriate.

The conductive layer 832 is formed with a single layer or stacked layersusing an element selected from molybdenum (Mo), aluminum (Al), tantalum(Ta), tungsten (W), titanium (Ti), copper (Cu), chromium (Cr), niobium(Nb), neodymium, scandium, nickel, and the like, or an alloy material ora chemical compound material containing any of the above elements as itsmain component (for example, tantalum nitride). Alternatively, theconductive layer 832 is formed using a semiconductor material typifiedby polycrystalline silicon doped with an impurity element such asphosphorus.

The following are preferable examples of the two-layer structure of theconductive layer 832: a two-layer structure in which a molybdenum filmis provided over an aluminum film, a two-layer structure in which amolybdenum film is provided over a copper film, a two-layer structure inwhich a titanium nitride film or a tantalum nitride film is providedover a copper film, a two-layer structure in which a titanium nitridefilm and a molybdenum film are stacked, a two-layer structure in which afilm of a copper-magnesium alloy containing oxygen and a copper film arestacked, a two-layer structure in which a film of a copper-manganesealloy containing oxygen and a copper film are stacked, a two-layerstructure in which a copper-manganese alloy film and a copper film arestacked, or the like. As a three-layer structure, it is preferable tostack a tungsten film or a tungsten nitride film, an alloy film ofaluminum and silicon or an alloy film of aluminum and titanium, and atitanium nitride film or a titanium film. By stacking a metal filmfunctioning as a barrier film over a film having low electricresistance, electric resistance can be low and diffusion of a metalelement from the metal film into a semiconductor film can be prevented.

Through the step of forming the conductive layer 832, a gate wiring (ascan line) and a capacitor wiring can also be formed at the same time.The scan line means a wiring for selecting a pixel, while the capacitorwiring means a wiring which is connected to one of electrodes of astorage capacitor in a pixel. However, one embodiment of the presentinvention is not limited thereto, and either one or both of the gatewiring and the capacitor wiring may be formed by a separate step fromthat of the conductive layer 832.

The insulating film 831 can be formed by a CVD method, a sputteringmethod, or the like.

Further, by the formation of a silicon oxide film by a CVD method usingan organosilane gas as the insulating film 831, the crystallinity of thesemiconductor film to be formed later can be increased, whereby theon-state current and the field-effect mobility of the transistor can beincreased. As an organosilane gas, a silicon-containing compound such astetraethoxysilane (TEOS) (chemical formula: Si(OC₂H₅)₄),tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC₂H₅)₃), ortrisdimethylaminosilane (SiH(N(CH₃)₂)₃) can be used.

The semiconductor layer 812 and the semiconductor layer 822 arepreferably formed using a crystalline silicon layer, but may be formedusing an amorphous silicon layer. The crystalline silicon layer isformed in such a manner that, after an amorphous silicon film is formed,the amorphous silicon film is crystallized by laser irradiation.Alternatively, after a metal film such as a Ni film is formed over anamorphous silicon film, the amorphous silicon film may be thermallycrystallized. Further alternatively, a crystalline silicon film may beformed by a CVD method. An impurity element can be introduced by an iondoping method, an ion implantation method, or the like with the use ofan n-type impurity element or a p-type impurity element to form theimpurity regions 817 and the impurity regions 827. As the n-typeimpurity element, phosphorus (P), arsenic (As), or the like can be used.As the p-type impurity element, boron (B), aluminum (Al), gallium (Ga),or the like can be used. Here, ion doping is performed using phosphorus(P) to form the n-channel transistors 811 and 821.

Conductive layers 818 and conductive layers 828 are source electrodesand drain electrodes. Each of the conductive layers 818 and theconductive layers 828 can be formed with a single layer structure or astacked-layer structure using an element selected from aluminum,tungsten, titanium, tantalum, molybdenum, nickel, and neodymium, or analloy containing a plurality of any of these elements. For example, aconductive layer that is formed using an alloy that contains a pluralityof any of the elements given above can be formed from an aluminum alloythat contains titanium, an aluminum alloy that contains neodymium, orthe like can be used. In the case of forming a stacked-layer structure,for example, a structure where an aluminum layer or the above-describedaluminum alloy layer is sandwiched between titanium layers can be used.Alternatively, crystalline silicon to which an impurity element whichserves as a donor is added may be used. Further, a stacked-layerstructure in which a film on the side that is in contact with thecrystalline silicon to which an impurity element serving as a donor isadded is formed using titanium, tantalum, molybdenum, tungsten, or anitride of any of these elements, and a layer of aluminum or an aluminumalloy is formed thereover may be formed. Alternatively, the conductivefilm may have a stacked-layer structure where aluminum or an aluminumalloy is sandwiched with titanium, tantalum, molybdenum, tungsten, ornitride of any of these elements, on an upper side and a lower sidethereof. The conductive layers 818 and the conductive layers 828 areformed by a CVD method, a sputtering method, or a vacuum evaporationmethod. Note that one of the conductive layer 818 and the conductivelayer 828 serves as a signal line as well as a source electrode or adrain electrode. However, one embodiment of the present invention is notlimited thereto, and a signal line may be provided separately from thesource and drain electrodes.

An insulating film 837 and an insulating film 838 are interlayerinsulating films. A conductive layer 806 is a pixel electrode. Theconductive layer 806 serves as an anode or a cathode in a light-emittingdevice. Here, an example is described in which the conductive layer 806is formed over the insulating film 838 provided over the conductivelayer 818, but one embodiment of the present invention is not limitedthereto. For example, the conductive layer 806 may be provided over theinsulating film 837.

As the insulating film 837 and the insulating film 838, insulating filmsof silicon oxide, silicon oxynitride, silicon nitride, or the like whichare formed by a CVD method, a sputtering method, or the like can beused. Alternatively, an organic resin such as an acrylic resin, apolyimide resin, a benzocyclobutene-based resin, a siloxane-based resin,a polyamide resin, or an epoxy resin can be used.

As the conductive layer 806, a transparent conductive film composed of alight-transmitting conductive material may be used, and indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, and the like can be used. Needless to say, it ispossible to use indium tin oxide, indium zinc oxide, indium tin oxidewith silicon oxide added, or the like. Furthermore, a material that hasa high work function, for example, an element selected from nickel (Ni),tungsten (W), chromium (Cr), platinum (Pt), zinc (Zn), tin (Sn), indium(In), or molybdenum (Mo), or an alloy material including any of themetal elements as its main component, for example, titanium nitride,titanium silicon nitride, tungsten silicide, tungsten nitride, tungstensilicide nitride, or niobium nitride may be used to form a single layerfilm or a film of stacked layers.

Reference numeral 851 denotes an alignment film. The alignment film 851can be formed using an organic resin such as polyimide. The thickness ofthe alignment film 851 is greater than or equal to 40 nm and less thanor equal to 100 nm, preferably greater than or equal to 50 nm and lessthan or equal to 90 nm. With such a thickness, the pretilt angle of aliquid crystal material can be made large, which can reducedisclination.

A film having a coloring property (hereinafter referred to as a coloringfilm 846) is formed on the substrate 842. The coloring film 846functions as a color filter. Further, a light-blocking film 844 adjacentto the coloring film 846 is formed on the substrate 842. Thelight-blocking film 844 functions as a black matrix. The coloring film846 is not necessarily provided in the case where the liquid crystaldisplay device is a monochrome display device, for example.

The coloring film 846 is a coloring film that transmits light in aspecific wavelength range. For example, a red (R) color filter fortransmitting light in a red wavelength range, a green (G) color filterfor transmitting light in a green wavelength range, a blue (B) colorfilter for transmitting light in a blue wavelength range, or the likecan be used.

The light-blocking film 844 preferably has a function of blocking lightin a particular wavelength range, and can be a metal film or an organicinsulating film including a black pigment.

An insulating film 848 is formed on the coloring film 846. Theinsulating film 848 functions as a planarization film or suppressesdiffusion of impurities in the coloring film 846 to the liquid crystalelement side.

A conductive layer 808 is formed on the insulating film 848. Theconductive layer 808 functions as the other of the pair of electrodes ofthe liquid crystal element in the pixel portion. Note that the alignmentfilm 851 is formed over the conductive layer 806 and an alignment film852 is formed on the conductive layer 808.

A liquid crystal layer 807 is formed between the conductive layer 806and the conductive layer 808. The liquid crystal layer 807 is sealedbetween the substrate 800 and the substrate 842 with the use of asealant (not illustrated). The sealant is preferably in contact with aninorganic material to prevent entry of moisture and the like from theoutside.

A spacer may be provided between the conductive layer 806 and theconductive layer 808 to maintain the thickness of the liquid crystallayer 807 (also referred to as a cell gap).

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Embodiment 3

In this embodiment, the driver circuit portion of the display devicedescribed in the above embodiments will be described.

FIG. 9 is a diagram of an entire gate driver circuit as one example of adriver circuit of a display device. A gate driver circuit 600 includes aplurality of shift register units 601, a shift register unit 602 whichis a dummy stage, demultiplexer circuits 603 electrically connected tothe shift register units 601, a demultiplexer circuit 604 electricallyconnected to the shift register unit 602, and signal lines transmittinga start pulse SP and clock signals (CLK1 to CLK8).

To the shift register unit 601 (here, description is made by using ashift register unit in the first stage), as shown in FIG. 10A, a setsignal LIN (here, the start pulse SP), a reset signal RIN, and clocksignals (here, CLK6 and CLK7) are input. FIG. 10B illustrates a specificcircuit structure example. The shift register unit 601 includes a firsttransistor 611 to a sixth transistor 616.

One of a source and a drain of the first transistor 611 is connected toa high power supply potential line VDD. The other of the source and thedrain of the first transistor 611 is connected to one of a source and adrain of the second transistor 612 and an input terminal FN1 of thedemultiplexer circuit 603. The set signal LIN is input to a gate of thefirst transistor 611. The other of the source and the drain of thesecond transistor 612 is connected to a low power supply potential lineVSS. A gate of the second transistor 612 is connected to an inputterminal FN2 of the demultiplexer circuit 603, one of a source and adrain of the fourth transistor 614, one of a source and a drain of thefifth transistor 615, and one of a source and a drain of the sixthtransistor 616. One of a source and a drain of the third transistor 613is connected to the high power supply potential line VDD. The other ofthe source and the drain of the third transistor 613 is connected to theother of the source and the drain of the fourth transistor 614. Theclock signal CLK7 is input to a gate of the third transistor 613. Theclock signal CLK6 is input to a gate of the fourth transistor 614. Theother of the source and the drain of the fifth transistor 615 isconnected to the low power supply potential line VSS. The set signal LINis input to a gate of the fifth transistor 615. The other of the sourceand the drain of the sixth transistor 616 is connected to the high powersupply potential line VDD. The reset signal RIN is input to a gate ofthe sixth transistor 616. Note that a portion in which the other of thesource and the drain of the first transistor 611 and the one of thesource and the drain of the second transistor 612 are electricallyconnected is referred to as a node FN1. A portion in which the gate ofthe second transistor 612, the one of the source and the drain of thefourth transistor 614, the one of the source and the drain of the fifthtransistor 615, and the one of the source and the drain of the sixthtransistor 616 are electrically connected is referred to as a node FN2.

The clock signals CLK6 and CLK7 are input to the shift register unit 601in the (8a+1)th stage (a is zero or a natural number). The clock signalsCLK3 and CLK4 are input to the shift register unit 601 in the (8a+2)thstage (a is zero or a natural number). The clock signals CLK1 and CLK8are input to the shift register unit 601 in the (8a+3)th stage (a iszero or a natural number). The clock signals CLK5 and CLK6 are input tothe shift register unit 601 in the (8a+4)th stage (a is zero or anatural number). The clock signals CLK2 and CLK3 are input to the shiftregister unit 601 in the (8a+5)th stage (a is zero or a natural number).The clock signals CLK7 and CLK8 are input to the shift register unit 601in the (8a+6)th stage (a is zero or a natural number).

The clock signals CLK4 and CLK5 are input to the shift register unit 601in the (8a+7)th stage (a is zero or a natural number). The clock signalsCLK1 and CLK2 are input to the shift register unit 601 in the 8(a+1)thstage (a is zero or a natural number).

The set signal LIN and clock signals (here, CLK3 and CLK4) are input tothe shift register unit 602 which is a dummy stage, as illustrated inFIG. 11A. FIG. 11B illustrates a specific circuit structure example. Theshift register unit 602 includes the first transistor 611 to the fifthtransistor 615.

The one of the source and the drain of the first transistor 611 isconnected to the high power supply potential line VDD. The other of thesource and the drain of the first transistor 611 is connected to the oneof the source and the drain of the second transistor 612 and the inputterminal FN1 of the demultiplexer circuit 604. The set signal LIN isinput to the gate of the first transistor 611. The other of the sourceand the drain of the second transistor 612 is connected to the low powersupply potential line VSS. The gate of the second transistor 612 isconnected to the input terminal FN2 of the demultiplexer circuit 604,the one of the source and the drain of the fourth transistor 614, andthe one of the source and the drain of the fifth transistor 615. The oneof the source and the drain of the third transistor 613 is connected tothe high power supply potential line VDD. The other of the source andthe drain of the third transistor 613 is connected to the other of thesource and the drain of the fourth transistor 614. The clock signal CLK4is input to the gate of the third transistor 613. The clock signal CLK3is input of the gate of the fourth transistor 614. The other of thesource and the drain of the fifth transistor 615 is connected to the lowpower supply potential line VSS. The set signal LIN is input to the gateof the fifth transistor 615. Note that a portion in which the other ofthe source and the drain of the first transistor 611 and the one of thesource and the drain of the second transistor 612 are electricallyconnected is referred to as the node FN1. A portion in which the gate ofthe second transistor 612, the one of the source and the drain of thefourth transistor 614, and the one of the source and the drain of thefifth transistor 615 are electrically connected is referred to as thenode FN2.

As illustrated in FIG. 12A and FIG. 13A, clock signals and outputsignals are input from the shift register unit 601 and the shiftregister unit 602 (signals input to the input terminal FN1 and the inputterminal FN2) to the demultiplexer circuit 603 and the demultiplexercircuit 604, and the demultiplexer circuit 603 and the demultiplexercircuit 604 output signals. FIG. 12B and FIG. 13B each illustrate aspecific circuit structure example. The demultiplexer circuit 603 andthe demultiplexer circuit 604 each include a buffer 605.

FIG. 14 illustrates one example of a specific circuit structure of thebuffer 605. A clock signal CLK (one of the clock signals CLK1 to CLK8)is input to one of a source and a drain of a seventh transistor 617. Theother of the source and the drain of the seventh transistor 617 isconnected to one of a source and a drain of an eighth transistor 618 andan output terminal. A gate of the seventh transistor 617 is connected tothe node FN1. The other of the source and the drain of the eighthtransistor 618 is connected to the low power supply potential line VSS.A gate of the eighth transistor 618 is connected to the node FN2.

A shift register unit may be a shift register unit 601 a which isillustrated in FIG. 15A and FIG. 15B and in which a transistor 621, atransistor 622, a transistor 623, and a capacitor 624 are added to theshift register unit 601. Note that a reset signal RES is input to a gateof the transistor 623.

Similarly, a shift register unit which is a dummy stage may be a shiftregister unit 602 a which is illustrated in FIG. 16A and FIG. 16B and inwhich the transistor 621, the transistor 622, the transistor 623, andthe capacitor 624 are added to the shift register unit 602. Note thatthe reset signal RES is input to the gate of the transistor 623.

To initialize the shift register unit, a pulse of the reset signal RESis input to turn on the transistor 623, so that the potential of thenode FN2 becomes equal to the potential of the high power supplypotential line VDD. The second transistor 612 and the transistor 621 areturned on with the potential of the node FN2, so that the potential ofthe node FN1 becomes equal to the potential of the low power supplypotential line VSS. Consequently, the shift register unit can beinitialized. Note that the reset signal RES is input to all of the shiftregister units using a common signal line.

As illustrated in FIG. 17A and FIG. 17B, the buffer 605 can be replacedwith a buffer 605 a further provided with a transistor 625 and acapacitor 619.

The capacitor serves as storage capacitor for holding charge.

In the shift register unit 601 in the first stage, the clock signalsCLK1 to CLK5 are input to the demultiplexer circuit 603 and thedemultiplexer circuit 603 outputs output signals OUT1 to OUT5.

The potential of the node FN2 is fixed to a high potential in a periodin which a gate selection output is not output, so that the secondtransistor 612 and the eighth transistor 618 are always on. In this way,the output is a low potential stably. However, in the case where thecutoff current (a drain current flowing at a gate voltage of 0 V) of thefifth transistor 615 is high, charge of the node FN2 leaks through thefifth transistor 615; therefore, charge needs to be regularlycompensated. Thus, the third transistor 613 and the fourth transistor614 are turned on with the clock signals CLK6 and CLK7, so that chargefor the node FN2 is supplied from the high power supply potential lineVDD. Note that a gate selection output period (the period in which thenode FN1 is at high potential) of the shift register unit 601 in thefirst stage is a period from the rising (set) of the start pulse SP tothe rising (reset) of the clock signal CLK7, which is described later.In the period, the gate selection output period and timing of regularcompensation of charge are set not to overlap each other with two clocksignals.

In the shift register unit 601 in the first stage, the clock signal CLK8is not input to anywhere. The clock signal is also provided to avoidoverlapping timing of regular compensation of charge.

Similarly, in the shift register unit 601 in the second stage, the clocksignals CLK1, CLK2, and CLK6 to CLK8 are input to the demultiplexercircuit 603, and the demultiplexer circuit 603 outputs the outputsignals OUT1 to OUT5. The clock signals CLK3 and CLK4 have a function ofregularly compensating charge. In the shift register unit 601 in thesecond stage, the clock signal CLK5 is not input to anywhere.

The same can be applied to the shift register units 601 in the third andthe following stages. In other words, one stage of the shift registerunit inputs five clock signals to the demultiplexer circuit 603, and thedemultiplexer circuit 603 outputs five output signals. Other two clocksignals have a function of regularly compensating charge and are inputto the shift register unit 601. The other clock signal is not input toanywhere.

The same is also applied to the shift register unit 602 which is a dummystage. The clock signals CLK1 and CLK2 are input to the demultiplexercircuit 604, and the demultiplexer circuit 604 outputs output signalsDUMOUT1 and DUMOUT2. The clock signals CLK3 and CLK4 have a function ofregularly compensating charge.

The number of clock signals are eight in this embodiment, but thepresent invention is not limited thereto. The number of clock signalsmay be any number as long as it is four or more. For example, when thenumber of clock signals is n, the number of output signals is (n−3)since three clock signals do not contribute to output signals.

In other words, by connecting n signal lines for transmitting clocksignals to one stage of the shift register unit, (n−3) output signalscan be output. The larger n becomes, the smaller the rate of signallines for transmitting clock signals which do not contribute to outputbecomes; accordingly, the area of the shift register unit part is smallcompared to a conventional structure in which one stage of a shiftregister unit outputs one output signal. Therefore, the width of thegate driver circuit 600 can be reduced.

Here, to narrow the width of the gate driver circuit 600 is brieflydescribed. FIG. 18A is a block diagram of a conventional gate drivercircuit. FIG. 18B is a block diagram of a gate driver circuit in thisembodiment.

In a conventional gate driver circuit illustrated in FIG. 18A, one stageof a shift register unit SR is connected to four signal lines CLK_LINEfor transmitting a clock signal and one buffer BUF outputs one signal.On the other hand, in the gate driver circuit in this embodimentillustrated in FIG. 18B, one shift register unit SR is connected toeight signal lines CLK_LINE for transmitting clock signals and fivebuffers BUF output five signals.

The gate driver circuit in this embodiment can have a smaller horizontallayout width of one shift register unit than that of the conventionalgate driver circuit. The vertical layout width increases because ofincreased buffers BUF (here, five times as much as the conventionalone), but the increase does not contribute to the bezel of the gatedriver circuit. The horizontal layout width of one shift register unitcan be reduced, so that the bezel can be narrower. In comparison withthe conventional one, the number of the signal lines CLK_LINE fortransmitting a clock signal is increased, and accordingly, loadcapacitance for each signal line CLK_LINE can be reduced. Therefore,even when the signal line CLK_LINE is set to thin to increase loadresistance, delay time is not changed (because time constant=loadcapacitance×load resistance). Accordingly, by making the width of thesignal line thin to obtain the same time constant, increase in layoutwidth can be prevented; therefore, even if the number of the signallines CLK_LINE is increased, the width of the gate driver circuit can bereduced.

Next, the operation of the gate driver circuit 600 is described withreference to a timing diagram in FIG. 19 . Here, high potentials of theset signal LIN, the reset signal MN, and the clock signals CLK1 to CLK8correspond to the potential of the high power supply potential line VDD,while low potentials of the set signal LIN, the reset signal MN, and theclock signals CLK1 to CLK8 correspond to the potential of the low powersupply potential line VSS.

In the driving method of the gate driver circuit 600 shown in FIG. 19 ,first, the start pulse SP is set to a high potential to turn on thefirst transistor 611 and the fifth transistor 615. Since the resetsignal RIN (the output signal OUT7) is a low potential, the sixthtransistor 616 is turned off. Since the clock signals CLK1 to CLK6 arelow potentials and the clock signals CLK7 and CLK8 are high potentials,the fourth transistor 614 and the seventh transistor 617 are turned offand the third transistor 613 is turned on.

At that time, the potential of the node FN1 has a value obtained bysubtracting the threshold voltage of the first transistor 611 from thepotential of the high power supply potential line VDD (VDD−Vth(611)),while the potential of the node FN2 becomes equal to the potential ofthe low power supply potential line VSS. Accordingly, the seventhtransistor 617 is turned on and the eighth transistor 618 is turned off,and thus, the output signals OUT1 to OUT5 are low potentials, as in theclock signals CLK1 to CLK5.

Then, the clock signal CLK7 is set to a low potential, so that the thirdtransistor 613 is turned off. Note that a high potential is held at anode in which the other of the source and the drain of the thirdtransistor 613 and the one of the source and the drain of the fourthtransistor 614 are electrically connected.

Next, the clock signal CLK1 changes from a low potential to a highpotential, and the potential of the node FN1 increases by a voltagecorresponding to the amplitude of the clock signal CLK1 by a bootstrapoperation. As a result, the seventh transistor 617 is turned on, and ahigh potential (the potential of the clock signal CLK1) is output as theoutput signal OUT1. Note that the bootstrap operation occurs similarlywhen the clock signals followed by the clock signal CLK2 changes from alow potential to a high potential. Next, the clock signal CLK8 becomes alow potential, but change does not occur because a signal of the clocksignal CLK8 is not used for the shift register unit 601 in the firststage. Then, the clock signal CLK2 becomes a high potential, and a highpotential is output as the output signal OUT2. After that, the clocksignal CLK1 becomes a low potential, and a low potential is output asthe output signal OUT1. The same can be applied to the followingoperation associated with the output signals OUT3 and OUT4. When theclock signal CLK5 becomes a high potential and the output signal OUT5becomes a high potential, the set signal LIN of the shift register unit601 in the second stage becomes a high potential.

In the shift register unit 601 in the first stage, when the clock signalCLK6 becomes a high potential, the fourth transistor 614 is turned on.Then, the clock signal CLK5 becomes a low potential, and a low potentialis output as the output signal OUT5.

In the shift register unit 601 in the second stage, the set signal LIN(the output signal OUT5) becomes a high potential, and the firsttransistor 611 and the fifth transistor 615 are turned on. Since thereset signal RIN (an output signal OUT12) is a low potential, the sixthtransistor 616 is turned off. Since the clock signals CLK1, CLK2, andCLK6 to CLK8 become a low potential and the clock signals CLK4 and CLK5become high potential, the fourth transistor 614 and the seventhtransistor 617 are turned off and the third transistor 613 is turned on.

At this time, the potential of the node FN1 has a value obtained bysubtracting the threshold voltage of the first transistor 611 from thepotential of the high power supply potential line VDD (VDD−Vth(611)),while the potential of the node FN2 becomes equal to the potential ofthe low power supply potential line VSS. Accordingly, the seventhtransistor 617 is turned on and the eighth transistor 618 is turned off,and thus, output signals OUT6 to OUT10 become a low potential, as in theclock signals CLK1, CLK2, and CLK6 to CLK8.

Next, the clock signal CLK4 becomes a low potential, and the thirdtransistor 613 is turned off. Note that a high potential is held at anode in which the other of the source and the drain of the thirdtransistor 613 and the one of the source and the drain of the fourthtransistor 614 are electrically connected.

Next, the clock signal CLK6 changes from a low potential to a highpotential, and the potential of the node FN1 increases by a voltagecorresponding to the amplitude of the clock signal CLK6 by a bootstrapoperation. As a result, the seventh transistor 617 is turned on, and ahigh potential (the potential of the clock signal CLK6) is output as theoutput signal OUT6. Next, the clock signal CLK5 becomes a low potential,but change does not occur because a signal of the clock signal CLK5 isnot used for the shift register unit 601 in the second stage. Then, theclock signal CLK7 becomes a high potential, and a high potential isoutput as the output signal OUT7.

At that time, in the shift register unit 601 in the first stage, thereset signal RIN (the output signal OUT7) becomes a high potential, andthe sixth transistor 616 is turned on, so that the potential of the nodeFN2 becomes equal to the potential of the high power supply potentialline VDD. The second transistor 612 is turned on with the potential ofthe node FN2, so that the potential of the node FN1 becomes thepotential of the low power supply potential line VSS and then is reset.

The shift register unit 601 in the second stage is driven like the shiftregister unit 601 in the first stage.

That is, the output signal OUT5(m−1) of the shift register unit 601 inthe (m−1)th stage is input as the set signal LIN of the shift registerunit 601 in the m-th stage (m is a natural number). The output signalOUT5(m+2) of the shift register unit 601 in the (m+1)th stage is inputas the reset signal RIN of the shift register unit 601 in the m-thstage. Note that the set signal LIN when m is 1 corresponds to the startpulse SP.

The shift register unit 602 which is a dummy stage is similar to theshift register unit 601. The reset signal RIN can be input to the shiftregister unit 601 in the final stage with the shift register unit 602.

Note that pulses of a clock signal and the next clock signal overlap byone third of the pulse width in this embodiment, but the presentinvention is not limited thereto. The overlap width may be any value aslong as it is half of the pulse width or less. The falling of the pulseof the clock signal and the rising of the pulse of the next clock signalmay be at the same timing. In that case, the gate selection outputperiod of the shift register unit 601 in the first stage is a periodfrom the rising (set) of the start pulse SP to the rising (reset) of theclock signal CLK6; accordingly, the number of clock signals needed forregularly compensating charge is only one.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

Embodiment 4

The semiconductor device of one embodiment of the present invention canbe used in a sensor that can detect proximity or touch of an object(e.g., a capacitive, a resistive, a surface acoustic wave, an infrared,and an optical touch sensor) and a radiographic image detection devicethat can obtain a medical radiographic image. The semiconductor devicewhich is one embodiment of the present invention can be applied to avariety of electronic appliances (including game machines). Examples ofelectronic appliances include a television device (also referred to astelevision or television receiver), a monitor of a computer or the like,a digital camera, a digital video camera, a digital photo frame, amobile phone, a portable game machine, a portable information terminal,an audio reproducing device, a game machine (e.g., a pachinko machine ora slot machine), and a game console, and the like. Examples of theseelectronic appliances are illustrated in FIGS. 7A to 7C.

FIG. 7A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. The housing 9001 issupported by four leg portions 9002. Further, a power cord 9005 forsupplying power is provided for the housing 9001.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9003. Thus, the display portion 9003 canhave high display quality.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her fingers or the like, the user can carryout operation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the display portion 9003 may function as a controldevice which controls the home appliances by operation on the screen.For example, with use of a semiconductor device having an image sensorfunction, the display portion 9003 can have a touch-input function.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 7B illustrates a television device 9100. In the television device9100, a display portion 9103 is incorporated in a housing 9101 and animage can be displayed on the display portion 9103. Note that thehousing 9101 is supported by a stand 9105 here.

The television device 9100 can be operated with an operation switch ofthe housing 9101 or a separate remote controller 9110. Channels andvolume can be controlled with an operation key 9109 of the remotecontroller 9110 so that an image displayed on the display portion 9103can be controlled. Furthermore, the remote controller 9110 may beprovided with a display portion 9107 for displaying data output from theremote controller 9110.

The television device 9100 illustrated in FIG. 7B is provided with areceiver, a modem, and the like. With the receiver, general televisionbroadcasts can be received in the television device 9100. Further, whenthe television device 9100 is connected to a communication network bywired or wireless connection via the modem, one-way (from a transmitterto a receiver) or two-way (between a transmitter and a receiver orbetween receivers) data communication can be performed.

Any of the semiconductor devices described in the above embodiments canbe used for the display portions 9103 and 9107. Thus, the televisiondevice can have high display quality.

FIG. 7C illustrates a computer 9200, which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

Any of the semiconductor devices described in the above embodiments canbe used for the display portion 9203. Thus, the computer 9200 can havehigh display quality.

FIGS. 8A and 8B illustrate a foldable tablet terminal. The tabletterminal is opened in FIG. 8A. The tablet terminal includes a housing9630, a display portion 9631 a, a display portion 9631 b, a display modeswitch 9034, a power switch 9035, a power saver switch 9036, a clasp9033, and an operation switch 9038.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9631 a and the display portion 9631 b.Thus, the display quality of the tablet terminal can be improved.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has a touchpanel function is illustrated as an example, the structure of thedisplay portion 9631 a is not limited thereto. The whole area of thedisplay portion 9631 a may have a touch screen function. For example,the whole area of the display portion 9631 a can display keyboardbuttons and serve as a touch screen while the display portion 9631 b canbe used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch screen region 9632 b. When a keyboard display switchingbutton 9639 displayed on the touch panel is touched with a finger, astylus, or the like, a keyboard can be displayed on the display portion9631 b.

Touch input can be performed concurrently on the touch screen regions9632 a and 9632 b.

The display-mode switching switch 9034 can switch display orientation(e.g., between landscape mode and portrait mode) and select a displaymode (switch between monochrome display and color display), for example.The power-saving-mode switching switch 9036 can control displayluminance in accordance with the amount of external light in use of thetablet terminal detected by an optical sensor incorporated in thetablet. The tablet terminal may include another detection device such asa sensor for detecting orientation (e.g., a gyroscope or an accelerationsensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 8A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

In FIG. 8B, the tablet terminal is folded and includes the housing 9630,a solar cell 9633, and a charge and discharge control circuit 9634. Notethat in FIG. 8B, an example in which the charge and discharge controlcircuit 9634 includes the battery 9635 and the DCDC converter 9636 isillustrated.

Since the tablet can be folded in two, the housing 9630 can be closedwhen not in use. Thus, the display portions 9631 a and 9631 b can beprotected, thereby providing a tablet with high endurance and highreliability for long-term use.

In addition, the tablet terminal illustrated in FIGS. 8A and 8B can havea function of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing the data displayed on the displayportion by touch input, a function of controlling processing by avariety of kinds of software (programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch screen, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630, so that thebattery 9635 can be charged efficiently. The use of a lithium ionbattery as the battery 9635 is advantageous in downsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 8B are described with reference to a blockdiagram of FIG. 8C. The solar cell 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are shown in FIG. 8C, and the battery 9635, the DCDCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge/discharge control circuit 9634 in FIG. 8B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar battery is raised or lowered by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. Then, when the power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 soas to be a voltage needed for the display portion 9631. In addition,when display on the display portion 9631 is not performed, the switchSW1 is turned off and a switch SW2 is turned on so that charge of thebattery 9635 may be performed.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation thereon, the battery 9635may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, the battery 9635 may be charged with anon-contact power transmission module that transmits and receives powerwirelessly (without contact) to charge the battery or with a combinationof other charging means.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments.

This application is based on Japanese Patent Application serial No.2013-144276 filed with Japan Patent Office on Jul. 10, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a substrate; a firstinsulating layer over the substrate; a second insulating layer over thesubstrate; a semiconductor layer comprising silicon over the firstinsulating layer and the second insulating layer; a third insulatinglayer over the semiconductor layer; a fourth insulating layer over thethird insulating layer; a gate electrode over the fourth insulatinglayer; an interlayer insulating film over the gate electrode; a firstconductive film provided in an opening of the interlayer insulatingfilm, the first conductive film having a stacked-layer structure; afifth insulating layer over and in contact with side surfaces and a topsurface of the first conductive film and over and in contact with theinterlayer insulating film; a second conductive film functioning as apixel electrode over and electrically connected to the first conductivefilm through an opening of the fifth insulating layer; a light-blockingfilm overlapping with the semiconductor layer, the gate electrode, andthe pixel electrode; a pair of LDD regions in the semiconductor layer;and a liquid crystal display element, wherein a bottom surface of thefifth insulating layer is provided below a top surface of the gateelectrode, wherein the second conductive film comprises a regionoverlapping with the semiconductor layer, wherein the gate electrodecomprises a region not overlapping with the second conductive film,wherein the gate electrode overlaps with a channel region of thesemiconductor layer, and wherein the light-blocking film overlaps withthe opening of the interlayer insulating film.
 2. A display devicecomprising: a substrate; a first insulating layer comprising siliconoxide over the substrate; a second insulating layer comprising siliconnitride over the substrate; a semiconductor layer comprisingpolycrystalline silicon, the semiconductor layer being over the firstinsulating layer and the second insulating layer; a third insulatinglayer comprising silicon oxide and functioning as a first gateinsulating layer, the third insulating layer being over thesemiconductor layer; a fourth insulating layer comprising siliconnitride and functioning as a second gate insulating layer, the fourthinsulating layer being over the third insulating layer; a gate electrodecomprising molybdenum over the fourth insulating layer; an interlayerinsulating film comprising one of silicon oxide and silicon nitride overthe gate electrode; a first conductive film provided in an opening ofthe interlayer insulating film, the first conductive film having astacked-layer structure comprising aluminum and titanium; a fifthinsulating layer comprising a resin over and in contact with sidesurfaces and a top surface of the first conductive film and over and incontact with the interlayer insulating film; a second conductive filmfunctioning as a pixel electrode over and electrically connected to thefirst conductive film through an opening of the fifth insulating layer;a light-blocking film overlapping with the semiconductor layer, the gateelectrode, and the pixel electrode; a pair of LDD regions in thesemiconductor layer; and a liquid crystal display element, wherein thelight-blocking film comprises a metal, wherein a bottom surface of thefifth insulating layer is provided below a top surface of the gateelectrode, wherein the second conductive film comprises a regionoverlapping with the semiconductor layer, wherein the gate electrodecomprises a region not overlapping with the second conductive film,wherein the second conductive film comprises indium tin oxide, whereinthe gate electrode overlaps with a channel region of the semiconductorlayer, wherein an end portion of the gate electrode has a tapered shape,and wherein the light-blocking film overlaps with the opening of theinterlayer insulating film.
 3. A display device comprising: a glasssubstrate; a first insulating layer comprising silicon oxide over theglass substrate; a second insulating layer comprising silicon nitrideover the glass substrate; a semiconductor layer comprisingpolycrystalline silicon, the semiconductor layer being over the firstinsulating layer and the second insulating layer; a third insulatinglayer comprising silicon oxide and functioning as a first gateinsulating layer, the third insulating layer being provided over thesemiconductor layer; a gate electrode comprising molybdenum over thethird insulating layer; an interlayer insulating film comprising one ofsilicon oxide and silicon nitride over the gate electrode; a firstconductive film provided in an opening of the interlayer insulating filmand the third insulating layer, the first conductive film having astacked-layer structure comprising aluminum and titanium; an organicinsulating film over and in contact with side surfaces and a top surfaceof the first conductive film and over and in contact with the interlayerinsulating film; a second conductive film functioning as a pixelelectrode over and electrically connected to the first conductive filmthrough an opening of the organic insulating film; a light-blocking filmoverlapping with the semiconductor layer, the gate electrode, and thepixel electrode; a pair of LDD regions in the semiconductor layer; and aliquid crystal display element, wherein the light-blocking filmcomprises a metal, wherein a bottom surface of the organic insulatingfilm is provided below a top surface of the gate electrode, wherein thesecond conductive film comprises a region overlapping with thesemiconductor layer, wherein the gate electrode comprises a region notoverlapping with the second conductive film, wherein the secondconductive film comprises indium tin oxide, wherein the gate electrodeoverlaps with a channel region of the semiconductor layer, and whereinan end portion of the gate electrode has a tapered shape.
 4. The displaydevice according to claim 1, further comprising: a sixth insulatinglayer over the second conductive film, and a region in which thesubstrate overlaps with the first insulating layer, the secondinsulating layer, the semiconductor layer, the third insulating layer,the interlayer insulating film, and the first conductive film without alayer comprising a metal provided therebetween.
 5. The display deviceaccording to claim 2, further comprising: a sixth insulating layer overthe second conductive film, and a region in which the substrate overlapswith the first insulating layer, the second insulating layer, thesemiconductor layer, the first gate insulating layer, the interlayerinsulating film, and the first conductive film without a layercomprising a metal provided therebetween.
 6. The display deviceaccording to claim 3, further comprising: a fourth insulating layer overthe second conductive film, and a region in which the glass substrateoverlaps with the first insulating layer, the second insulating layer,the semiconductor layer, the first gate insulating layer, the interlayerinsulating film, and the first conductive film without a layercomprising a metal provided therebetween.
 7. The display deviceaccording to claim 1, further comprising: an alignment film comprising aresin over the second conductive film; and a third conductive filmcomprising indium tin oxide over the second conductive film and thefifth insulating layer.
 8. The display device according to claim 2,further comprising: an alignment film comprising a resin over the secondconductive film; and a third conductive film comprising indium tin oxideover the second conductive film and the fifth insulating layer.
 9. Thedisplay device according to claim 3, further comprising: an alignmentfilm comprising a resin over the second conductive film; and a thirdconductive film comprising indium tin oxide over the second conductivefilm and the organic insulating film.
 10. The display device accordingto claim 3, wherein the light-blocking film overlaps with the opening ofthe interlayer insulating film.